It is known that concentrations of a few parts per million of soluble cyanides such as sodium cyanide are toxic to the microflora and microfauna which comprise the food-chain of higher forms of aquatic life such as fish, waterfoul, and eventually man. For this reason the United States Environmental Protection Agency (E.P.A.) has enacted strict laws to regulate the amount of soluble cyanides which may be discharged from any source into natural waters.
For certain industrial operations, such as the extraction of gold and silver from their ores, soluble cyanide compounds such as sodium cyanide or potassium cyanide are essential reagents used in the extraction process. In earlier years, mining and other industrial companies traditionally discharged their waste waters, containing sometimes as much as 50-100 parts per million (ppm) of soluble cyanide, into streams or rivers. It was assumed that the relatively small concentrations of soluble cyanides in the waste waters would be greatly diluted, dissipated and inactivated by the natural stream of river waters.
Numerous studies by ecologists, limnologists, and environmental scientists have demonstrated that concentrations of free, chemically uncomplexed, cyanide ion (CN.sup.-) as low as 1 PPM are toxic to microflora and microfauna comprising the food-chain of fish and other animals.
For operations such as mining, electroplating, and similar industries which produce large volumes of waste waters containing soluble cyanides in concentrations in the range 1-50 ppm or more, the E.P.A. has enacted regulations which prohibit the discharge of waste waters that contain more than 0.02 ppm (1 part in 50 million) to such cyanides.
Compliance with this extremely low concentration of cyanide in industrial waste waters which are discharged to the environment has presented enormous problems to the industries that must meet such standards.
One solution for cyanide removal is aeration. However, aeration of acidified solutions containing free cyanide ion results in only limited removal of cyanide as gaseous hydrogen cyanide. This method is not effective for the removal of complex cyanides with metals such as zinc, nickel, copper, cobalt, and iron among others. In acidified and aerated solutions, these complexes gradually decompose and free cyanide ion increases exponentially.
The use of ferrous sulfate to precipitate soluble cyanide ion as the very insoluble compound, Prussian blue (ferric ferrocyanide) has been known for many years. This process has been shown to be effective in reducing the concentration of free or complexed cyanide ion from relatively high initial concentrations (e.g. 100, 500, 1000 ppm) to very low concentrations of total cyanide in the supernatant solution. Unfortunately, the supernatant solution over the Prussian blue precipitate is found always to contain approximately 0.5-3.0 ppm cyanide ion, depending upon the conditions of treatment of the solution with ferrous ion. This concentration of cyanide is well above that which is allowed by current E.P.A. regulations in waste waters.
Treatment of the solution resulting from Prussian blue precipitation by passage through suitable ion exchange resins has met with only partial or limited success. Although some ion exchange resins can reduce the total cyanide concentration from the initial 0.5-3.0 ppm to less than 0.02 ppm, industrial practice has demonstrated that the efficiency of removal of total cyanide ion rapidly deteriorates as the active adsorption sites on the ion exchange resin become covered. The result is that while some ion exchange resins initially have the ability to reduce the total cyanide concentration in the supernatant solution from Prussian blue precipitation from approximately 0.5-3.0 ppm to less than 0.02 ppm, it is found that the efficiency of these resins falls off rapidly, and the legally permitted upper limit of 0.02 ppm cyanide is soon exceeded.
A further disadvantage of ion exchange resins is their need for regeneration, to desorb the adsorbed complex ions resulting from the treatment of the original solution with ferrous ion. While ferrocyanide and ferricyanide ions can usually be stripped from the resins fairly easily by the passage of suitable concentrated eluting solutions, other complex cyanides (e.g. cuprocyanide, cupricyanide, cobalticyanide) present major problems of removal, requiring extremely long and industrially impractical contact times with the eluting solutions. In many cases it has been found that not all of the adsorbed complex cyanide ions can be removed by eluting solutions. This, of course, progressively reduces the adsorption sites and consequently the efficiency of the regenerated resin.
Another method of cyanide-waste treatment is by alkali-chlorination. In this process, cyanide is first oxidized by chlorine to the cyanate and then is either further oxidized, again using chlorine, to carbon dioxide and nitrogen or the cyanate is hydrolyzed to yield the same products using an acid such as sulfuric acid. This is a relatively costly process and does not directly result in recovery of the metals contained in the waste stream.
Electrolytic oxidation of relatively concentrated cyanide wastes is also possible and is commercially practiced. Electrolytic decomposition processes destroy the cyanide without formation of other toxic compounds and additionally recover much of the metal content as a cathode deposit. This technique, however, is not practical for use in the treatment of dilute solutions such as rinse waters.
In another approach, cyanides contained in waste streams are precipitated to form insoluble metal cyanides. This is illustrated by the Zabban patent, U.S. Pat. No. 2,845,330. Zabban uses a mixture of copper sulfate and sodium sulfite to precipitate insoluble metallic cyanides from waste streams. The principal disadvantage to this method is that it requires special and rather expensive reagents to destroy the cyanides and precipitate the metals.
Yet another method for destruction of the cyanide in waste solutions involves exposing the solution to gamma radiation. The radiation results in rupture of the C.tbd.N triple bonds to thereby convert the cyanide ions into non-toxic byproducts, which could be safely discharged into streams or otherwise disposed of. Although such radiation method may give good results in the destruction of cyanide ions, plating and metal finishing shops ordinarily do not have the necessary radiation equipment or apparatus available to practice the method and such equipment can only be obtained by a considerable monetary expenditure.
Furthermore, polymers have been disclosed for the removal of metals from fluid streams. Examples include the use of poly(dithiocarbamates) in U.S. Pat. No. 5,510,040; poly(ethylene dichloride/ammonia) in U.S. Pat. No. 5,346,627; condensations polymers of higher alkyl halides with polyamines in U.S. Pat. No. 3,932,274; condensation polymers of higher fatty acids with polyamines in U.S. Pat. No. 4,054,516; poly(alpha hydroxyacrylates) in U.S. Pat. No. 3,839,215; poly(isocyanurates) in U.S. Pat. No. 3,893,916.
Moreover, efforts to destroy free cyanide in waste solutions in the past have also involved the addition of Fe.sup.+++ ions to the solution to form and iron-cyanide complex with the cyanide tightly bound in the complex. However, this method suffers from the cyanide radical still being intact after the completion of the method, and hence significant toxicity still being present in the solution. What is urgently needed, therefore, is a process that will remove free or complex cyanide ions from solution as a stable precipitate and which will leave no detectable soluble cyanide species in solution. Such a process should also be rapid, economically feasible and industrially practicable. The present invention provides a process which attains these criteria.
Among the objects of this invention are: to provide a process by the use of which an aqueous solution containing complexed cyanide ions is treated chemically to reduce the concentration of such ions to analytically undetectable levels; to provide a process by the application of which an aqueous solution containing cyanide ions which are complexed with metal ions as coordination compounds are treated to reduce the concentration of such complexed cyanide ions to analytically undetectable levels in an industrially practicable and economic manner; to provide a process by the use of which complexed cyanide ions are treated to reduce the concentration of such cyanide ions to analytically undetectable levels and to produce a solution which is not toxic to aquatic or terrestrial organisms; to provide a process for removal of undesirable oxoanion contaminants from an aqueous stream, and to provide a process the application of which is environmentally acceptable.
I have discovered a class of compounds which can effectively meet all of the aforementioned objects.